Generally, a protective film is used for protecting the surface of metal products or plastic sheets. As an adhesive for the protective film, an acrylic adhesive has been widely used due to its weather-resistance and transparency. Also as the acrylic adhesive, an adhesive, in which a copolymer comprising an alkyl (meth)acrylate and a crosslinkable monomer is crosslinked with polyisocyanate compound, melamine compound or epoxy compound, has been mainly used.
Concerning adherents of the surface protective film such as polarizers, plastic sheets, electronic appliances, vehicles, and the like, it is very important to protect the adherents from static electricity as well as inflow of foreign material such as dust. Therefore, an anti-static property, together with a adhesive force, is very important in the surface protective film.
In general, static electricity means frictional electrification generated by rubbing two objects or peeling electrification generated by separating two adhered objects. Such static electricity may cause suction of foreign substances such as dust, electrostatic destruction of a device, malfunction of a instrument, fire, and so forth. In particular, the demand of liquid crystal displays (LCDs) has been remarkably increased due to wide spread of computers, expansion of liquid crystal TVs or multi-functional mobile phones, etc. As each assembly device is integrated, and becomes vulnerable to static electricity, it is more important to inhibit generation of static electricity. Also, as the size of the LCDs increases, the size of a polarizer used in manufacturing of the LCD device also increases. Moreover, with the speed-up of processing, static electricity is excessively generated during peeling of a protective film adhered to the polarizer, and then affecting the orientation of liquid crystal in the LCD device and thus causing image detect.
To prevent generation of static electricity, a method of forming an anti-static layer on an outer surface of a polarizer or a protective film substrate layer was proposed, but the effect was not good enough and could not prevent generation of static electricity fundamentally.
Therefore, the anti-static property is needed for the adhesive itself to prevent generation of static electricity. Conventional methods for providing the anti-static property of an adhesive include a method of adding a substance having a conductive component such as a conductive metal powder or a carbon particle to resin and a method of adding a surfactant-type ionic or nonionic substance. However, when the anti-static property is implemented using the additives, a large amount of additives has to be added and thus the additive is migrated to the surface of the pressure-sensitive adhesive, and then degrading a pressure-sensitive adhesive property.
Japanese Patent Laid-Open Publication No. 1993-140519 discloses a method for suppressing static electricity through an addition of ethylene oxide-modified phthalic acid dioctyl plasticizer. However, this method has disadvantages that the plasticizer is migrated to the surface of a polarizer.
Korean Laid-Open Patent Publication No. 2004-30919 discloses a method for suppressing static electricity by adding an organic salt, but this method needs a large amount of expensive organic salt. Korean Laid-Open Patent Publication No. 2006-128659 discloses a method using a chelating agent capable of forming a bond with a metal salt, and alkali metal salt. However, this method is also disadvantageous in that low-rate peel strength is reduced due to a large amount of additives.